1. Field of the Invention
This invention relates to semiconductor laser diode design and construction. In particular, this invention relates to design and construction techniques for providing enhanced laser beam characteristics from semiconductor lasers.
2. Description of the Prior Art
The use of conventional semiconductor lasers has been limited by many factors, one of which is the type of beam produced. Although generally similar to the beam from a conventional optical laser, the conventional semiconductor laser beam is anamorphic and astigmatic, that is, the laser beam is nonsymetrical and possesses intermediate foci.
Conventional semiconductor laser design and construction techniques have not been able to reduce or prevent beam anamorphism and astigmatism. Such conventional approaches utilize measurements of emitted beam energy and spot size as criteria for semiconductor laser design. Circularity of the semiconductor laser beam has been achieved only with post diode fixes, such as external optics.
The conventional diode-injection, or gain-guided, laser consists of a tiny chip of semiconductor material, such as gallium arsenide (GaAs) typically between 0.3 and 1 mm long, with even smaller transverse dimensions. The chip is cut from heavily n-doped material after p-type impurities have been diffused into the top of the material. The top layer becomes p-type with a thin, planar p-n junction created a short distance below the top surface. If the electrical contact to the diode is shaped into a narrow strip running the length of the diode, the profile of the injected carriers provides a weak, complex waveguide that confines the light laterally. This mechanism is called gain-guiding. Such gain-guided semiconductor lasers are highly astigmatic and have strong instabilities.
Gain guided techniques have been improved by the introduction of dielectric waveguide structures in the lateral direction. Since the light in these structures is guided by variations in the real refractive index of the various materials, the corresponding devices are known as index-guided lasers. Index-guided lasers support only the fundamental transverse and lateral modes to produce a stable, single-mode beam with significantly reduced astigmatism as compared to gain-guided semiconductor lasers.
These techniques result, however, in semiconductor laser designs which produce only a small fraction of the useful laser energy of which they are capable. Laser beams produced by such design and construction techniques are astigmatic and anamorphic and are therefore limited in useful beam energy.